Femtosecond laser pulses had been applied broadly in various fields such as biological, medical, processing, communication, defense, and others. Femtosecond lasers and relative technologies have also been developed quickly. Recently, hot research fields, such as femtosecond chemistry, femtosecond nonlinear optical microscopy imaging of chemical and biological materials, are all based on femtosecond lasers. Attosecond laser pulse generation, X-ray laser, laboratory astrophysics, laser acceleration of electrons and protons, and other strong-field laser physics are all taking the advantage of femtosecond laser pulses as a research tool. In contrast to the nanosecond and picosecond lasers, femtosecond laser processing can get much more refined and smooth surface shape. Then it was widely used in the field of femtosecond laser micromachining. Femtosecond laser pulses has also recently been used for ophthalmic lens cutting operation, which greatly improves the quality and safety of the kind of surgery.
In the application of the laser, the pulse shape and the pulse width of the femtosecond laser pulse are important optical parameters. Real-time measurement or monitoring of these parameters are necessary in many experiments. Therefore, a simple, convenient, and effective method and apparatus for laser pulse measurement and real-time monitoring is important and promotes the development and application of femtosecond laser technology.
The technique for femtosecond laser pulse width measurement is evolving as the development of femtosecond laser technology. Currently, the most commonly used methods include autocorrelation method, See R. Trebino, Frequency-Resolved Optical Grating: The Measurement of Ultrashort Laser Pulses, Kluwer Academic Publishers (2000), frequency-resolved optical gating (FROG) method, See R. Trebino et al., “Measuring ultrashort laser pulses in the time frequency domain using frequency-resolved optical gating,” Rev. Sci. Instrum.68 (9), 3277-3295 (1997), and spectral phase interferometry for direct electric-field reconstruction (SPIDER) method, See C. Iaconis et al., “Spectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulses,” Opt. Lett. 23 (10), 792-794 (1998). Autocorrelation method is simple in the principles and structure but can not obtain the phase information of femtosecond laser pulses. FROG and SPIDER are used to get the pulse phase. However, FROG method usually need a long time to rebuild the pulse. SPIDER method usually requires a nonlinear optical crystal to convert the generated measurement signal. Because of the phase matching conditions in nonlinear optical crystals, each apparatus can only be adapted to a particular spectral range, thus limiting the application of the method in a wide spectrum range.
Recently, cross-polarized wave (XPW) generation, see A. Jullien et al., “Spectral broadening and pulse duration reduction during cross polarized wave generation: influence of the quadratic spectral phase,” Appl. Phys. B 87 (4), 595-601(2007), is used as a reference light for self-referenced spectral interferometry (SRSI) method, see T. Oksenhendler et al., “Self-referenced spectral interferometry,” Appl. Phys. B 99 (1), 7-12 (2010), to measure the femtosecond laser pulse. In this method, one incident beam is used without being divided into two beams. In the calculation, only three simple iterative calculations are needed to quickly obtain the spectra and spectral phase of the measured laser pulse, which is by far the most simple and convenient method. However, it requires the polarizer in the method. Then, the method is only valid for a particular wavelength, which also limits the application of the method and apparatus within a specific spectral range. The dispersion of the polarizer element also restricts the shortest pulse duration to be measured on 10 fs level. Recently, the self-diffraction effect based SRSI method has been used with the polarizer and relative restriction. In the method, the beam to be measured is divided into three beams. The current setup of the method is somewhat complex. See J. Liu et al., “Self-referenced spectral interferometry based on self-diffraction effect,” J. Opt. Soc. Am. B 29 (I): 29-34 (2012).